Plant and method for vacuum degassing liquid steel

ABSTRACT

The invention relates to a plant for vacuum degassing liquid steel, comprising: at least one vacuum chamber 2, suitable to temporarily receive liquid steel inside it; a vacuum generation system 10, connected to said at least one vacuum chamber 2 via an intake duct 20. The vacuum generation system 10 comprises at least two compression stages connected together in series, of which: a first compression stage 11 works closer to the aforesaid at least one vacuum chamber and is composed of one or more screw pumps 110; and a second compression stage 12 works farther with respect to said at least one vacuum chamber 2 to bring the gases at least to atmospheric pressure and is composed of one or more liquid ring pumps 120.

FIELD OF APPLICATION

The present invention relates to a plant and method for vacuum degassingliquid steel.

The plant and the method according to the invention can be used forvacuum degassing with either the VD (Vacuum Degassing) technique or theVOD (Vacuum Oxygen Decarburisation) technique and for all applicationswhere a vacuum treatment of liquid steel is required.

STATE OF THE ART

The vacuum degassing process (called for simplicity VD/VOD, from theEnglish “Vacuum Degassing ” and “Vacuum Oxygen Decarburisation”)respectively is a steel process that has as its main objective that ofproducing steels that meet high quality standards and stainless steels.

The vacuum treatment makes it possible to achieve. extremely low levelsof sulphur, hydrogen and nitrogen, improve the micro and macro purity ofthe steel, and, in the case of VOD, decarbonise (reduce the carboncontent) the steel.

Generally, VD/VOD systems are designed to operate round the clock andeach treatment lasts from 35 to 120 minutes depending on theproductivity required of the plant and on operating practices.

Although various plant solutions exist aimed at meeting specificrequirements (available installation space, required productivity),normally a steel degassing plant consists of the following components,as shown in the general diagram in FIG. 1:

a vacuum chamber A, airtight to the outside, inside which the ladle Lcontaining the liquid steel is housed.

a vacuum generator B, i.e. a system able to aspirate gases until apressure of less than 1 mbar absolute is achieved inside the vacuumchamber.

an intake line C which places the vacuum Chamber A in communication withthe vacuum generator B and the latter with the stack D through which thegases generated by the process are discharged;

devices aimed at managing the process, installed along the intake line(valves illustrated below and gas pressure and temperature measuringinstruments, a heat exchanger for cooling the process gas in output fromthe vacuum chamber);

a dust separation unit F generally composed of a cyclone (to removelarger particles) and a filter (to retain smaller particles);

a gas insufflation system G, usually argon, in some cases even nitrogen,for the agitation of the liquid steel and removal of impurities withinit;

an insufflation system E of inert gas into the intake duct in order tomanage the foamy slag, raising the pressure inside the isolated system;

where the vacuum decarburisation of steel (VOD) is also provided for, anoxygen injector controlled by an auxiliary system is installed on thelid of the vacuum chamber.

The vacuum chamber A consists of a lid A1 and a tank A2. Depending onwhich is the fixed part and which is the mobile part there are two typesof construction: a “wheeled lid” when the tank is fixed and the lidmobile, and a “wheeled tank” in the opposite case.

Generally speaking, depending on the operating principle the vacuumgenerator B may be of two types: with a steam ejector/liquid ring(technical solution more popular in the past) or mechanical pumps(technology becoming more widespread recently).

As shown in FIG. 1, the devices for managing the process and installedalong the intake line C usually comprise a valve V1 to return the vacuumchamber to atmospheric pressure, a main valve V2 to isolate the vacuumchamber from the vacuum generator, a valve V3 for insufflating nitrogento control the process.

The degassing plant in general is divided into two parts by the mainvalve V2. There are thus two volumes: a tank volume and a retainedvolume.

The tank volume is returned to atmospheric pressure after every vacuumtreatment by opening the valve V1 which effectively places the vacuumchamber in communication with the external environment. The retainedvolume, instead, is generally kept in a vacuum thanks to the main valveV2, which keeps it isolated from the external environment. Themaintenance of the vacuum in the retained volume makes it possible toshorten the time required to lower the pressure in the system by usingit as a “plenum chamber” equalising the pressure between the tank andretained volume at the moment of opening the main valve V2. It should benoted that the tank is at atmospheric pressure before opening the mainvalve V2.

Generally, a vacuum degassing process comprises the following steps:

positioning the ladle containing liquid steel inside the vacuum chamberand closing the lid;

aspirating the gases contained inside the plant volume to achieve therequired vacuum level (typically <1 mbar);

permanence at the operating pressure for the time deemed appropriate(typically from 15 to 25 minutes) to achieve the metallurgicalobjectives;

restoration of the atmospheric pressure inside the vacuum chamber(opening of valve V1), and refining of the chemical analysis byadditions of materials in precise quantities.

The intake gas is composed primarily of air up to a pressure of about100-150 mbar, then of metal vapours, hydrogen and nitrogen coming fromthe steel. The suction capacity of the vacuum generation systemautomatically adjusts throughout the range of pressures. The operator isrequested to perform an adjustment only in the case of abnormal chemicalreactions inside the vacuum chamber (especially in cases of foaming ofthe slag, present in the ladle with the molten steel, to avoidingleakages of incandescent material from the ladle itself).

The control of the entire process passes through the movement of the lidand/or tank and the command of the automatic cycles for the adjustmentof the operating conditions of the system (i.e. of the working points ofthe vacuum generator in order to control the pressure inside the vacuumchamber).

It is also known that during the entire degassing process a large amountof dust is produced.

The material constituting the dust derives mainly from the evaporationof metal elements present in the liquid bath, subsequently condensedalong the intake line and the filter, from the reaction between thesteel and the refractory and, to a lesser extent, from iron-alloys andscorifiers.

During a VD process approximately 0.1-0.2 kg of dust is produced pertonne of treated steel: during a complete treatment up to 20-40 kg maybe produced (considering for example a ladle of a capacity of 200 tonnesof liquid steel). A typical analysis of the dust composition reveals asignificant content of Zn, MgO, CaO, Pb, Mn.

In the VOD process (“Vacuum Oxygen Decarburisation”, a vacuum processwith insufflation of oxygen to achieve low levels of carbon in theliquid steel) the amount of dust generated may reach 800-1000 kg (for200 tonnes of liquid steel).

It is essential to have an effective dust collection system to preservethe vacuum generator from wear or clogging phenomena as well as to avoiddust emissions into the atmosphere.

If pressure filtration is required, a cyclone separator (tangential airintake) and a bag filter are installed in series on the intake line.However, filter installations also exist with an integrated cyclone.

Typically, the dust from these processes, because of its composition,burns very easily in the presence of oxygen. For this reason the bagfilters (which are currently the most common technology for suchapplications) require frequent and efficient cleaning, which istypically done automatically after every treatment, by blowing inert gas(nitrogen) in counter-flow to the canvas bag, a technology known as“reverse pulse jet”.

Aside from environmental requirements concerning atmospheric emissions,the need to install elements for dust abatement (bag filters andcyclone) or not, is determined by the degree of dust tolerated by thevacuum system to be installed.

To date, there are two vacuum generation technologies based oncompletely different operating principles: mechanical pumps and steamejector systems.

Vacuum Generation with Mechanical Pumps

In the terminology commonly used in the steel industry, mechanical pumpvacuum generation refers to a vacuum generator which provides for theinstallation in series of lobe type blowers (root pumps) and screw pumps(screw pump) as illustrated in FIG. 2. In this case the screw pumps arealso called “pre-vacuum pumps”.

As a general principle, since each of these machines performs acompression of the aspirated gas, a compression “stage” is spoken ofreferring to one or more machines operating in the same pressure rangebetween intake and discharge.

The current most widespread plant solutions consist of a series of ascrew pump and at least two root pumps in series, as shown in FIG. 2.

The stages are conventionally named in ascending numerical order (stage1, . . . , stage n) starting from those closest to the vacuum chamber A.The last stage is that which finally discharges the gases into theatmosphere (pre-vacuum stage). Each stage may consist of several pumpsconnected in parallel, as shown in FIG. 3.

The criterion determining the arrangement in series is as follows: screwpumps are capable of operating with very high compression ratios (up to1:1000) but with low volumetric flow rates; root pumps instead are ableto dispose of large volumes of gas, but do not permit high compressionratios (typically about 1:6).

In typical VD/VOD installations, operatively, the screw pump alone isable to maintain a pressure of not less than 20-50 mbar inside thevacuum chamber, downloading the gases into the atmosphere. In order toachieve a higher degree of depression (<1 mbar) the upstreaminstallation of at least two stages of root pumps is required. Thelatter, thanks to the type of construction, (a double inner chamberalternately liberated and obstructed by the rotating lobes) are mosteffective in moving very rarefied gases, which gases at low pressuresare.

In short, in stable operating conditions (i.e. disregarding the initialevacuation transient of the vacuum chamber starting from atmosphericpressure), the early-stage root pumps aspirate the process gases at verylow pressures (<1 mbar) and deliver them to the screw pumps in thepressure range in which the latter operate with higher compressioneffidiency.

The main drawback of using mechanical pumps in a configuration asdescribed above is related to the need to perform filtration of theaspirated gases in order to retain the solid particulate which couldblock and/or damage the rotating mechanical bodies (seizure) andpossibly contaminate the lubrication oil (contained in the gear chamberin the case of deterioration of the gaskets). The root pumps—while notmeant for use in a pulverulent environment—would theoretically becapable of treating pulverulent gases without running into operationalproblems of seizure. In the long run however oil contamination problemswould arise. The biggest problem relates to the screw pumps which wouldbe forced —without filtration—to treat the pulverulent gases dischargedby the root pumps, incurring in the aforesaid seizure problems andleading to immediate blocking of the system.

In describing the typical operating conditions of a vacuum generationsystem with mechanical pumps reference is generally made to 4 automaticcycles which determine the functioning of the main devices installed(valves, filter, pumps):

activation cycle of the system: the pumps are started and the volumeuntil the main valve (“retained volume”) is evacuated reaching a finalpressure typically <5 mbar; the vacuum chamber at this stage remains atatmospheric pressure and the pumps are kept at a minimum rotation speed;

Degassing cycle: the main valve opens to equalise pressure in acontrolled manner between the vacuum chamber and the “retained” volume;the slow equalisation is designed not to overstress the system from amechanical point of view (pumps and filterbags) and avoid theinstantaneous and violent oxidation of the pyrophoric dust remaining onthe surface of the filter bags after the previous treatments. The pumpsgradually speed up to reach the maximum rotation speed. During thelowering of the pressure the vacuum level in the system can becontrolled by slowing down/by-passing the pumps or by insufflatingnitrogen. Typically the process pressure (<1 mbar) is reached in 6-8minutes.

Stop vacuum cycle: when degassing is complete, the main valve closes andthe vacuum chamber is returned to atmospheric pressure (permitting thesubsequent opening of the lid and addition of materials).

Cleaning cycle: with the pumps isolated, the filter bags are cleaned bymeans of a system of nitrogen blows, the cleaning cycle being thenfollowed by a dust discharge cycle as necessary.

Once the cleaning cycle of the bags is compete, the retained volume isagain evacuated (up to pressures<5 mbar) preparing the system for thenext degassing cycle.

The cleaning of the filter bags is a crucial aspect for the performanceof the mechanical pump system because:

an excessive accumulation of dust on the bags increases the pressurelosses through the filter, limiting the minimum pressure which can bereached inside the vacuum chamber;

possible damage of the bags causes large quantities of dust to reach thepumps. The operation of the system may thus be jeopardised if thecleaning and maintenance of filters is not properly conducted (correctsetting of the wash cycle with nitrogen, regular inspections of the bags. . . ).

Vacuum Generation with Ejector Pumps

Ejector vacuum generators use as a propellent fluid the superheatedsteam generated in a boiler or coming from other sources. As a result ofthe acceleration of the steam and the architecture of the ejector, theprocess gas is aspirated and compressed.

Each ejector is sized to compress a given quantity of gas, achieving aspecific ratio between the intake and discharge pressure (typically tothe order of 1:5/1:15). To operate between the pressure required by theprocess (1 mbar) and atmospheric pressure (1000 mbar) several differentejectors operating in series are therefore required.

In this case too, in the arrangement in series, each ejector isconsidered as a compression “stage”. A stage may however be composed ofseveral ejectors in parallel to increase the suction capacity of thesystem at higher pressures (typically required during the evacuationphase of the vacuum chamber).

FIG. 4 shows a plant layout of a typical ejector pumping station whereS1, S2, S3 and S4 indicate ejector stages, C1, C2 and C3 inter-stagecondensers and P a collection tank or “hot pit”. The S3 and S4 stages,in this particular case, consist of pairs of A/B ejectors operating inparallel. The activation sequence of the individual stages is usuallycontrolled by the pressure reached by the vacuum chamber, and is asfollows (with reference to FIG. 4): S4-S3-S2-S1.

To ensure maximum efficiency of the ejector system (disposal of themaximum flow of process gas), heat exchangers are installed in serieswith the ejectors to condense the steam contained in the main gas flow.

The steam, in fact, acts only as a propeller to aspirate the processgases and condense as the pressure increases and the temperaturedecreases.

The steam is thus made to condense inside the “condensers” which aredrained into a tank, called a “hot pit”.

It is clear that, in the absence of filtering systems upstream of theejector groups, the condensed water has a higher concentration of dust,thus requiring appropriate wastewater treatment plants and maintenanceoperations for the disposal of the sludge channelled into the “hot pit”.

FIG. 4 shows a diagram of a typical ejector consisting of fourcompression stages.

A variation of this diagram provides that the fourth stage, oralternatively a possible fifth stage, consists of a liquid ring pump inplace of an ejector. This solution is generally preferred in systemswith limited steam availability or where required by plant or processrequirements (limited space for installation, need to operate stably atpressures above 100 mbar for VOD systems).

The liquid ring pump is a mechanical, centrifugal-type pump in which thecompression of the gas, by means of its confinement in a variable(gradually reduced) volume, is consequential to the rotation of a liquidring generated by a centrifugal effect of a rotor, eccentric to thecasing (body) of said pump.

With the exception of the composition of the pumping system and of thedust abatement group connected thereto, the operation of anejector/liquid ring plant passes through operational sequences entirelysimilar to those described for the mechanical pumps.

For the ejector systems it is not necessary, for the purpose ofprotecting the pumping system, to abate the dust to the extent ofrequiring the installation of a bag filter since it lacks the geometrictolerances required by the mechanical system, typical of root or screwpumps.

On the other hand, in some systems, to minimise maintenance (cleaning ofthe ejectors and hot pit water treatment) a cyclone or even a bag filterwith related automatic cleaning system may be installed.

Lastly, it is to be noted that in the absence of filter elements, alarge amount of dust is retained by the injected steam and by the waterof possible liquid ring pumps. The condensed steam between one ejectorstage and another helps to retain some of the dust generated during theprocess. The condensed water is drained, as mentioned above, into the“hot pit” (indicated as P in FIG. 4). Also the possible liquid ring inits contact with the process gas helps to retain part of the residualdust. It follows that in a liquid ring/ejector system the amount of dustcontained in the gases discharged to the stack is very low.

In conclusion, the main difference as regards the system layout betweenejector systems and mechanical pump systems lies in the presence of abag filter (with all the auxiliary elements for cleaning the bags anddischarging the dust), required in the latter case to preserve theintegrity of the machines.

The main limitation of injector vacuum generation systems lies in theircomplexity and high plant and running costs.

PRESENTATION OF THE INVENTION

Consequently, the purpose of the present invention is to eliminateentirely or in part the drawbacks of the prior art mentioned above, byproviding a plant and method for vacuum degassing liquid steel combiningthe engineering/operational simplicity of a mechanical pump plant withthe possibility to operate without filter systems of an ejector plant.

A further purpose of the present invention is to make available a plantfor vacuum degassing liquid steel which is operatively more reliable.

A further purpose of the present invention is to make available a plantfor vacuum degassing liquid steel which is cheaper to run.

A further purpose of the present invention is to make available a plantfor vacuum degassing liquid steel which is at least comparable toconventional systems with mechanical pumps, in terms of plant costs.

BRIEF DESCRIPTION OF THE DRAWINGS

The technical characteristics of the invention, according to theaforesaid purposes, can be seen clearly from the contents of thefollowing claims and the advantages of the same will be more clearlycomprehensible from the detailed description below, made with referenceto the appended drawings, showing one or more embodiments by way ofnon-limiting examples, wherein:

FIG. 1 shows a general diagram of a steel degassing plant;

FIG. 2 shows a general diagram of a conventional vacuum generationsystem with mechanical pumps of the root and screw type;

FIG. 3 shows a general diagram of a conventional vacuum generationsystem with mechanical pumps of the root and screw type, with each stagecomposed of several pumps in parallel;

FIG. 4 is a diagram of a conventional ejector vacuum generation system;

FIG. 5 shows a general diagram of a liquid steel vacuum degassing plantaccording to a preferred embodiment of the present invention;

FIG. 6 shows a general diagram of a liquid steel vacuum degassing plantaccording to an alternative embodiment of the present invention;

FIG. 7 shows a general diagram of the vacuum generation system in aliquid steel vacuum degassing plant according to a preferred embodimentof the present invention; and

FIG. 8 shows a general diagram of a liquid ring pump.

DETAILED DESCRIPTION

With reference to the appended drawings reference numeral 1 globallydenotes a plant for vacuum degassing liquid steel according to theinvention.

The plant 1 according to the invention can be used for vacuum degassingwith either the VD (Vacuum Degassing) technique or the VOD (VacuumOxygen Decarburisation) technique and for all applications where avacuum treatment of liquid steel is required.

Here and henceforth in the description and the claims, reference will bemade to the vacuum degassing plant of liquid steel 1 in conditions ofuse.

According to a general embodiment of the invention, the plant for vacuumdegassing liquid steel comprises:

at least one vacuum chamber 2, suitable to temporarily receive liquidsteel inside it; and

a vacuum generation system 10, connected to the aforesaid at least onevacuum chamber 2 via an intake duct 20.

The vacuum chamber 2 may be of any type suitable for the purpose.

Preferably, the vacuum chamber 2 is configured so that the liquid steelis brought inside via a ladle L, but it may also be used directly toreceive the liquid steel.

In the first case, as shown in FIGS. 5 and 6, the vacuum chamber 2comprises a tank 3, which defines the volume of the chamber 2 and issuitable to receive therein the ladle L, and a lid 4 suitable to sealthe tank 3 tight when the ladle L is housed therein. The vacuum chambermay be of the “wheeled lid” type when the tank is fixed and the lidmobile or of the “wheeled tank” type in the opposite case.

Advantageously, as shown in FIGS. 5 and 6, the vacuum chamber may befitted with an insufflation system 30 of a washing gas, in some caseseven nitrogen, for the agitation of the liquid steel and removal ofimpurities within it. In particular, this insufflation system 30 isdesigned to feed one or more porous septums located on the bottom of theladle.

In the second case, according to an embodiment not illustrated in theappended Figures, the vacuum chamber 2 may be configured to directlyhouse within it the liquid steel acording to an RH process. In thiscase, the liquid steel is transferred temporarily from the ladle insidethe chamber. To such purpose the vacuum chamber is connected to a ladlevia two ducts: a delivery duct through which the molten steel from theladle is driven by the difference in pressure inside the vacuum chamber,and a return duct, through which the treated molten steel flows backfrom the vacuum chamber inside the ladle.

According to the invention, as shown in FIGS. 5 and 6, the vacuumgeneration system 10 comprises at least two compression stages connectedto each other in series, of which:

a first compression stage 11 works closer to the aforesaid at least onevacuum chamber 2 and consists of one or more screw pumps 110; and

a second compression stage 12 works farther away from the aforesaid atleast one vacuum chamber 2 to bring the gas at least to atmosphericpressure and consists of one or more liquid ring pumps 120.

The aforesaid one or more screw pumps 110 are sized to be able tooperate with compression ratios not exceeding 1:12 if the dischargepressure is atmospheric, and with compression ratios not exceeding 1:200if the discharge pressure is comprised between 50 and 120 mbar absolute.

As will be specified below, the aforesaid one or more screw pumps 110are thus sized in a radically different manner to conventional screwpumps.

Preferably the aforesaid one or more screw pumps 110 are sized to beable to operate with compression ratios comprised between 1:3 and 1:10if the discharge pressure is atmospheric and, if the discharge pressureis between and 120 mbar absolute, with compression ratios of between1:25 and 1:200, and preferably between 1:70 and 1:90.

Thanks to the fact of operating in the aforesaid compression ratioranges, the aforesaid one or more screw pumps 110 are sized imposinginternal tolerances (rotor/rotor and rotor/case), much higher than thoseprovided for in the conventional screw pumps used as pre-vacuum stagesas described above. This way, the aforesaid screw pumps 110 are able tooperate in direct contact with pulverulent gases with highconcentrations of dust without contraindications for the movingmechanical parts and thus without incurring in the typical problems ofscrew pumps used as pre-vacuum stages in conventional, mechanicaldegassing systems.

This is made possible by the fact that according to the invention thescrew pumps are used at stages closer to the vacuum chamber and by thechoice to operate such pumps in the aforesaid compression ranges.

Operationally, the work of compressing the gas is completed by theaforesaid one or more liquid ring pumps which define the secondcompression stage (final), further away from the vacuum chamber,exploiting the fact that the liquid ring pumps are insensitive to dust.

Advantageously, the liquid ring pumps also perform an important functionof retaining the solid particles dragged along by the main flow of thegases. The pump service water is therefore used to trap the dustgenerated by the degassing process and then collect it in a singlepoint. This way the dust emission at a possible discharge stack 40 isminimised, ensuring low environmental impact.

Thanks to the invention the vacuum generation system 10 is thus able toaspirate directly from the aforesaid at least one vacuum chamber 2 gasescontaining dust in high concentrations, without the contraindicationstypical of a conventional mechanical pump system.

Conventionally, contrary to the provisions of the present invention,screw pumps are instead used in the vacuum generation systems ofdegassing plants to define the compression stages furthest away from thevacuum chamber A. These pumps (pre-vacuum), despite operating withcompression ratios between 1:1 and 1:50, and preferably between 1:2 and1:40, are designed to work with compression ratios up to 1:1000 withdischarge at atmospheric pressure. Conversely, the screw pumps 110according to the invention are sized to operate at maximum compressionratios of 1:12 with discharge at atmospheric pressure. The traditionalscrew pumps must therefore be built with very strict internal tolerances(rotor/rotor and rotor/case). This makes them particularly sensitive tothe presence of dust in the gases treated.

Thanks to the present invention, it is therefore possible on the onehand to liberate the design of a liquid steel degassing plant from theinstallation of a filtration device (usually a bag filter) required inthe case of mechanical pumps, and on the other to drastically reduce theplant costs entailed by a conventional steam ejector system.

Advantageously, the vacuum generation system 10 is sized to bring thevacuum chamber 2 to a degree of vacuum between 0.2 and 5 mbar, andpreferably between 0.5 and 1:5 mbar. As a result, the vacuum generationsystem 10 is sized to generate total compression ratios between 1:5,000and 1:200.

As regards the sizing of the vacuum generation system 10 according tothe present invention, the possible combinations in terms of number ofscrew pumps 110 and liquid ring pumps 120 are dictated by the designchoices from time to time made so as to minimise the number of machinesinstalled to get the level of performance required by the process, i.e.evacuation times of the vacuum chamber limited and degree of finalvacuum approximately <1 mbar.

Advantageously, the vacuum generation system 10 may comprise one or moreintermediate compression stages, positioned in series between the firststage 11 and the second stage 12 and each composed of one or more screwpumps 110 having similar characteristics to those of the first stage 11.

The term “similar characteristics” is taken to mean that said one ormore screw pumps of the intermediate stages are sized to operate in thesame compression ranges as the screw pumps of the first stages, thusmaking it possible to adopt internal tolerances (rotor/rotor androtor/case) much higher than those provided for in conventional screwpumps. The size of the screw pumps of the intermediate stages may be thesame or different to that of the screw pumps of the first stages. Thechoice of size is dictated by the sizing of the vacuum generationsystem.

One or more of the aforesaid compression stages (first, second orintermediate) may each consist of two or more pumps connected inparallel.

According to embodiments not shown in the appended figures, the vacuumgeneration system may consist of two or more parallel pumping modules,each of which is composed at least of a first compression stage 11 withscrew pumps and a second compression stage 12 with liquid ring pumps.

The total number of pumps installed per module and the number of modulesis defined in the design phase with the objective of optimising theinstallation and minimising the consumption of auxiliary elements(water, nitrogen, electricity).

Advantageously, modular configurations may be adopted for the vacuumgeneration system 10, i.e. separable into units installed in parallel,or “hybrid” installations where the pumps are grouped on two stageswithout modularity.

Preferably, the vacuum generation system 10 can be isolated from therest of the system by closing appropriate isolation valves installedimmediately upstream of the pumps.

Preferably, as shown in FIGS. 5 and 6, the intake duct 20 comprises aby-pass duct 21 able to exclude from the gas flow the compressor stagesformed of the screw pumps 110. This solution can be adopted both in thecase of a modular structure, and a non-modular structure.

Operatively, as will be resumed below, the presence of the aforesaidby-pass 21 may be used to exclude the screw pumps from functioning insome stages of the degassing process.

Preferably, each of the screw pumps 110 used in the degassing plant 1according to the invention comprises two screw rotors, kinematicallysynchronised with each other via an electric axis.

For the connection and synchronization of the two screw rotors thesepumps do not use the conventional “mechanical axis”, where an enginetransmits movement to a screw rotor while the other rotor isdragged/synchronised by means of a series of gears in oil bath.

The term “electric axis” means the software synchronisation of a pair ofengines by means of an inverter (one for each screw) and a pair ofencoders. The software instantly manages the parameters of the twoinverters so that the rotors are constantly synchronised. Furthermorethe two encoders control the angular deviation of the axes of the screwrotors, so that these are perfectly parallel to each other.

Operatively, any functional anomaly (e.g. internal friction due to dustbuild-up) results in an increase in torque and current absorption of themotors (or of one of them) and, consequently, a possible deviation inthe angular speed of the rotors. Advantageously, the software can act onthe speed in real time until equilibrium is restored, avoiding stressesand overheating of the pump.

Compared to a solution with a mechanical axis, this electric axisconfiguration does not require oil for the lubrication of the gears. Theabsence of lubrication oil is an advantage. In fact, due to the possibledifference in pressure between the compression chamber (lower pressure)and possible (concurrent) gear chambers (higher pressure), the oil canbe aspirated into the process gas, mixing with the dust and generatingobstructions. Similarly, in certain operating phases, dusty gases caninundate the gear chambers polluting the oil.

The liquid ring pumps 120 used in the degassing plant 1 according to thepresent invention are of the type known per se and their operation istherefore well, known to a technician of the sector. A detaileddescription of the same is therefore not provided but merely referenceto a number of concepts useful for introducing some particular elements.

In particular, the liquid ring pumps used in the present invention mayhave the structure shown in FIG. 8.

As shown in FIG. 8, a liquid ring pump compresses the process gas G′between an eccentric vane rotor 121 and a ring 122 of water, calledservice water W. Operatively, the dust carried by the process gas G′necessarily comes into contact with the service water W which acts as acollector. The pump 120 ejects the compressed gas G″ together with aminimum amount of pulverulent service water. The mixture G″+W of gas andpulverulent water reaches a separator 123 which separates the gas (nowat atmospheric pressure and directed to the stack) from the “dirty”water which is collected in the lower part of the separator 123.Advantageously, a replenishment 124 of, the water W is provided tooffset the losses from evaporation.

More specifically, the service water W can be handled in two ways: in anopen circuit or closed circuit.

With closed-circuit management the water W is recirculated until thesaturation limit of dust, at which the pump performance drops. At thispoint all the service water W is discharged and replaced with cleanwater.

With open circuit management, the water is continuously discharged fromthe separator (through the opening 125 illustrated in FIG. 8), while aline of clean water 124 continuously tops up the service circuit of theliquid ring pump.

Advantageously, the resulting water contains dust which is now inert andcan be handled in two different ways.

According to a first method, the pulverulent water is collected in adecanting bath with an overflow which leads to a second bath. From herethe pulverulent water is sent on to a water treatment plant, by means ofcentrifugal pumps, and treated therein in the conventional way.

According to a second method, as shown schematically in FIG. 7, thepulverulent water leaving the separator can be filtered on site usingknown methods.

Advantageously, as shown in FIG. 7, the plant 1 comprises an auxiliaryunit 50, which, in addition to replenishing the water dispersed by theliquid ring pumps in the process gases, separates the dust contained inthe water and recirculates it to the pump.

A continuous cycle operation guarantees both the controlled removal ofthe dust (avoiding internal build-up) and optimal operation of theliquid ring pump 120 thanks to the cooling and cleaning of the top-upwater.

The auxiliary unit 50 may be centralised or located on board of eachliquid ring pump or module, maintaining however the same functions.

Alternatively to the aforesaid auxiliary unit 50, the plant 1 maycomprise at least one continuous replacement device of the service waterused by the liquid ring pump, without recirculation, with non-returnablewater.

Advantageously, as shown in FIGS. 5 and 6, in the section comprisedbetween the vacuum chamber 2 and the vacuum generation system 10 theintake duct 20 comprises a connection branch 28 to the atmosphereequipped with a first control valve 23. This first control valve 23 isopened at the end of the degassing process to return the vacuum chamber2 to atmospheric pressure before taking out the treated liquid steel.

Advantageously, as shown in FIGS. 5 and 6, in the section comprisedbetween the vacuum chamber 2 and the vacuum generation system 10 theintake duct 20 may comprise a connection branch 29 to a tank (not shown)containing inert gas (nitrogen or argon), equipped with a second controlvalve 24. The inert gas can be insufflated by opening the second valve24 in order to manage the foamy slag, raising the internal pressure.

According to a preferred embodiment illustrated in FIG. 5, the degassingplant 1 does not comprise a filtration device of the gases, which leavethe vacuum chamber 2 and have to pass through the vacuum generationsystem 10. Regardless of the concentration level of the dust in saidgases, the gases in output from the vacuum chamber 2 are aspirateddirectly by the vacuum generation system without a preventive gasfiltration step. As noted previously, this is possible thanks to thepresent invention.

According to an alternative embodiment illustrated in FIG. 6 thedegassing plant 1 may comprise at least one filtration device 25 of thegases leaving the vacuum chamber 2 and passing through the vacuumgeneration system 10. Such a filtration device 25 is arranged betweenthe vacuum chamber 2 and the vacuum generation system 10.

Operatively, the gases exiting the vacuum chamber 2, before beingaspirated by the vacuum generation system, are subjected to filtrationin order to abate at least partially the dust content present in thegases. Thanks to the present invention, the abatement of the dust may bepartial and bland, given that the possible presence of dust does notaffect the operation of the vacuum generation system 10. The preventivefiltration step may be provided so as to optimise the management of dustin the system, reducing the load of dust to be managed by means of theliquid ring pumps.

The aforesaid filtration device 25 may consist of a bag filter, acyclone or of an integrated bag filter and cyclone system.

In particular, according to the alternative embodiment illustrated inFIG. 6, the plant 1 comprises at least an isolation valve 22 which isinstalled on the intake duct 20 between the vacuum chamber 2 and thefiltration device 25. Such isolation valve 22 is placed downstream ofthe branching point of the intake duct 20 into the aforesaid connectionbranch 28 to the atmosphere. The isolation valve 22 divides the plant 1into two parts, thus identifying two volumes. A first part comprises thevacuum chamber (tank volume); the second part comprises the filtrationdevice and the vacuum generation system (retained volume).

Operatively, the tank volume is returned to atmospheric pressure afterevery vacuum treatment by opening the aforesaid first control valve 23which places the vacuum chamber in communication with the externalenvironment. The retained volume may, instead, be always kept in avacuum thanks to the isolation valve 22 which effectively keeps itairtight. The maintenance of the vacuum of the retained volume makes itpossible to shorten the time required to lower the pressure in thesystem by using it as a “plenum chamber” equalising the pressure betweenthe tank and retained volume at the moment of opening the isolationvalve 22.

The presence of the isolation valve 22 is preferred in the case in whichthe plant 1 is equipped with a filtration device 25 (in particular if itis a bag filter) as shown in FIG. 6. In this case, the retained volumeis very high due to the presence of the filtration device.

Advantageously, in the case in which the plant 1 is not equipped with afiltration device 25 (see FIG. 5), the isolation valve 22 need not beinstalled, since, in the absence of the filtration device, the retainedvolume is reduced and therefore the advantages associated withmaintaining said volume in a vacuum are limited.

Advantageously, in the case in which the plant is used for the vacuumdegassing with VOD (Vacuum Oxygen Decarburisation) technique it maycomprise a heat exchanger (not shown in the appended figures) forcooling the process gases. In fact, with the VOD technique, as a resultof the injection of oxygen and consequent decarburisation of the steel,the temperatures involved increase significantly. The heat exchangershould be placed upstream of the possible filtration device 22 anddownstream of the possible isolation valve (if present), in the secondpart of the system (retained volume).

The present invention also relates to a method for vacuum degassingliquid steel.

In particular, the method according to the invention may be implementedin a degassing system according to the invention, in particular asdescribed above. The parts in common with the plant 1 described abovehave been indicated using the same alpha-numerical references.

According to a general embodiment of the invention, the method forvacuum degassing liquid steel comprises the following operating steps:

a) providing at least one vacuum chamber 2 suitable to temporarilyreceive liquid steel inside it;

b) placing liquid steel in said vacuum chamber 2;

c) evacuating the vacuum chamber 2 through a vacuum generation system 10creating in said chamber a predefined degree of vacuum and maintainingit for a predetermined period of time so as to complete the operation ofdegassing the liquid steel; and

d) bringing again the vacuum chamber 2 to atmospheric pressure andwithdrawing the degassed liquid steel.

According to the invention, the vacuum evacuation step c) is conductedby means of a vacuum generation system 10 comprising at least twocompression stages connected together in series, of which:

a first compression stage 11 works closer to the aforesaid at least onevacuum chamber 2 and consists of one or more screw pumps 110; and

a second compression stage 12 works farther away from the aforesaid atleast one vacuum chamber 2 to bring the gas at least to atmosphericpressure and consists of one or more liquid ring pumps 120.

The aforesaid one or more screw pumps 110 are sized to be able tooperate with compression ratios not exceeding 1:12, if the dischargepressure is atmospheric, and with compression ratios not exceeding1:200, if the discharge pressure is comprised between 50 and 120 mbarabsolute.

Preferably, the aforesaid one or more screw pumps 110 are sized to beable to operate with compression ratios comprised between 1:3 and 1:10if the discharge pressure is atmospheric, and, if the discharge pressureis between 50 and 120 mbar absolute, with compression ratios of between1:25 and 1:200, and preferably between 1:70 and 1:90.

Preferably, in the aforesaid evacuation step c), the vacuum chamber 2 isbrought to work at a degree of vacuum between 0.2 and 5 mbar, andpreferably between 0.5 and 1.5 mbar.

According to a preferred embodiment of the method, the evacuation stepc) provides for the direct aspiration of the gases from the vacuumchamber 2 through the aforesaid vacuum generation system 10 without apreventive filtration step of the gases, independently of the level ofdust concentration in the gases themselves.

According to an alternative embodiment of the method, the evacuationstep c) provides for the aspiration of the gases from the vacuum chamber2 through the aforesaid vacuum generation system 10 with a preventivefiltration step of the gases, to reduce the dust concentration in saidgases before their passage through the vacuum generation system 10.

Preferably, the evacuation step c) comprises:

an initial evacuation step c1) wherein the vacuum chamber 2 is broughtfrom atmospheric pressure up to about 300 mbar using only the liquidring pumps of the vacuum generation system 10; and

a final evacuation step c2), wherein the vacuum chamber 2 is broughtfrom the pressure of about 300 mbar to the predefined degree of vacuum,also using the screw pumps.

This operating mode makes it possible to minimise the amount of dustwhich the screw pumps must handle, to the benefit of the operation ofsuch pumps. This operating mode takes advantage of the presence of theby-pass 21 which is present on the intake duct and which permits theexclusion of the screw pumps from the passage of the gases.

Advantageously, during the evacuation step c) the suction capacity ofthe vacuum generation system 10 can be varied to reduce any phenomena offoaming of the slag in the liquid steel. The suction capacity is variedby slowing or excluding one or more of the pumps of the vacuumgeneration system 10, preferably the liquid ring pumps 120.

Preferably, the above change in suction capacity is carried out when theinternal pressure of the vacuum chamber 2 is between 300 mbar and 1mbar, i.e. during the final evacuation step c2).

Preferably, the method according to the invention comprises a step f) oftreating the service water used by the aforesaid one or more liquid ringpumps 120. This treatment step f) is carried out preferably during theevacuation step c). The treatment consists of filtering the dust fromthe water or of continuously replacing the water.

Advantageously, the method comprises a step e) of mixing the moltensteel at least during the evacuation step c), in particular byinsufflating inert gases into said steel.

The invention permits numerous advantages to be achieved, in partalready described.

The plant 1 for vacuum degassing liquid steel according to the inventioncombines the plant simplicity of a mechanical pump plant with thepossibility to operate without the filter systems of an ejector plant.

Thanks to the invention it, is therefore possible to minimise theequipment installed in a degassing plant.

This way it ensures greater flexibility in the design phase (layout andauxiliaries) as well as allowing operation of the system whileminimising maintenance costs and possible repairs of the parts mostsubject to wear in a conventional system. In particular, it reducesperiodic inspections and eliminates the need to replace the filter bags.

The plant 1 for vacuum degassing liquid steel according to the inventionis therefore:

operatively more reliable; and

cheaper to run.

In terms of plant costs, the plant 1 for vacuum degassing liquid steelis at least comparable to conventional systems with mechanical pumps andcertainly less expensive than conventional systems with ejectors.

The advantages set forth above for the plant 1 according to theinvention extend to the degassing method according to the invention.

The invention thus conceived thereby achieves the intended objectives.

Obviously, its practical embodiments may assume forms and configurationsdifferent from those described while remaining within the sphere ofprotection of the invention.

Furthermore, all the details may be replaced by technically equivalentelements and the dimensions, shapes and materials used may be any asneeded.

1-20. (canceled)
 21. Plant for vacuum degassing liquid steel,comprising: at least one vacuum chamber (2), suitable to temporarilyreceive liquid steel inside it; a vacuum generation system (10),connected to said at least one vacuum chamber (2) via an intake duct(20), characterised in that the vacuum generation system (10) comprisesat least two compression stages connected together in series, of which afirst compression stage (11) works closer to said at least one vacuumchamber (2) and is formed by one or more screw pumps (110), and a secondcompression stage (12) works farther with respect to said at least onevacuum chamber (2) to bring the gases at least to atmospheric pressureand is formed by one or more liquid ring pumps (120) and in that saidone or more screw pumps (110) are dimensioned to be able to operate withcompression ratios not exceeding 1:12, if the discharge pressure isatmospheric, and with compression ratios not exceeding 1:200, if thedischarge pressure is comprised between 50 and 120 mbar absolute. 22.Plant according to claim 21, wherein said one or more screw pumps (110)are dimensioned to be able to work with compression ratios comprisedbetween 1:3 to and 1:10, if the discharge pressure is atmospheric, and,if the discharge pressure is between 50 and 120 mbar absolute, withcompression ratios comprised between 1:25 and 1:200, and preferablybetween 1:70 and 1:90.
 23. Plant according to claim 21, wherein thevacuum generation system (10) is dimensioned to bring the vacuum chamber(2) to a degree of vacuum between 0.2 and 5 mbar, and preferably between0.5 and 1.5 mbar.
 24. Plant according to claim 21, wherein the vacuumgeneration system (10) comprises at least one intermediate compressionstage, which is positioned between the first stage (11) and the secondstage (12) and is connected to them in series, said intermediatecompression stage being formed by one or more screw pumps (110) havingsimilar characteristics to those of the first stage (11).
 25. Plantaccording to claim 21, wherein one or more of said compression stagesare each formed by two or more pumps connected in parallel.
 26. Plantaccording to claim 21, wherein the intake duct (20) comprises a by-passduct (21) able to exclude from the gas flow the compressor stages formedby screw pumps (110).
 27. Plant according to claim 21, wherein eachscrew pump (110) comprises two screw rotors, kinematically synchronisedwith each other via electric axis.
 28. Plant according to claim 21,comprising at least one filtration device of the water used by said oneor more liquid ring pumps (120) suitable to remove dust accumulated inthe water itself during operation of the pump or a replacement device ofthe water itself.
 29. Plant according to claim 21, wherein, in thesection comprised between the vacuum chamber (2) and the vacuumgeneration system (10) the intake duct (20) comprises a connectionbranch (28) to the atmosphere equipped with a control valve (23). 30.Plant according to claim 21, comprising at least one gas filtrationdevice (25) positioned between the vacuum chamber (2) and the vacuumgeneration system (10).
 31. Plant according to claim 21, wherein, in thesection comprised between the vacuum chamber (2) and the vacuumgeneration system (10) the intake duct (20) comprises a connectionbranch (28) to the atmosphere equipped with a control valve (23) andwherein said plant comprises at least one gas filtration device (25)positioned between the vacuum chamber (2) and the vacuum generationsystem (10), said plant comprising at least one shut-off valve (22) thatis installed in said intake duct (20) between the vacuum chamber (2) andthe filtration device (25), downstream of the branching point of theconnection branch (28) to the atmosphere.
 32. Method for vacuumdegassing liquid steel, comprising the following operating steps: a)providing at least one vacuum chamber (2) suitable to temporarilyreceive liquid steel inside it; b) placing liquid steel in said vacuumchamber (2); c) evacuating the vacuum chamber (2) through a vacuumgeneration system (10) creating in said chamber a predefined degree ofvacuum and maintaining it for a predetermined period of time so as tocomplete the operation of degassing the liquid steel; d) bringing againthe vacuum chamber (2) to atmospheric pressure and withdrawing thedegassed liquid steel; characterised in that the vacuum evacuation stepc) is conducted by means of a vacuum generation system (10) comprisingat least two compression stages connected together in series, of which afirst compression stage (11) works closer to said at least one vacuumchamber (2) and is formed by one or more screw pumps (110), and a secondcompression stage (12) works farther with respect to said at least onevacuum chamber (2) to bring the gases at least to atmospheric pressureand is formed by one or more liquid ring pumps (120) and in that saidone or more screw pumps (110) are dimensioned to be able to operate withcompression ratios not exceeding 1:12, if the discharge pressure isatmospheric, and with compression ratios not exceeding 1:200, if thedischarge pressure is comprised between 50 and 120 mbar absolute. 33.Method according to claim 32, wherein said one or more screw pumps (110)are dimensioned to be able to operate with compression ratios comprisedbetween 1:3 and 1:10, if the discharge pressure is atmospheric, and, ifthe discharge pressure is between 50 and 120 mbar absolute, withcompression ratios of between 1:25 and 1:200, and preferably between1:70 and 1:90.
 34. Method according to claim 33, wherein, in evacuationstep c), the vacuum chamber (2) is brought to working at a degree ofvacuum between 0.2 and 5 mbar, and preferably between 0.5 and 1.5 mbar.35. Method according to claim 32, wherein said evacuation step c)provides for the direct aspiration of the gases from said vacuum chamber(2) through the said vacuum generation system without a preventivefiltration step of the gases, independently of the level of dustconcentration in the gases themselves.
 36. Method according to claim 32,wherein said evacuation step c) provides for the aspiration of the gasesfrom said vacuum chamber (2) through said vacuum generation system witha preventive filtration step of the gases, to reduce the dustconcentration in the gases themselves before their passage through thevacuum generation system (10).
 37. Method according to claim 32, whereinevacuation step c) comprises: an initial evacuation step c1) wherein thevacuum chamber (2) is brought from atmospheric pressure up to about 300mbar using only the liquid ring pumps (120) of the vacuum generationsystem (10); and a final evacuation step c2) wherein the vacuum chamber(2) is brought from the pressure of about 300 mbar to the predefineddegree of vacuum also using the screw pumps (110).
 38. Method accordingto claim 32, wherein during evacuation step c), the aspiration capacityof the vacuum generation system (10) is varied to reduce foamingphenomena of the slag in the liquid steel, the aspiration capacity beingvaried by slowing or excluding one or more of the pumps (110, 120) ofthe vacuum generation system (10), preferably the liquid ring pumps(120), said variation of aspiration capacity being preferably carriedout when the internal pressure of the vacuum chamber (2) is between 300mbar and 1 mbar.
 39. Method according to claim 32, comprising atreatment step f) of the water used by said one or more liquid ringpumps (120), said step being preferably carried out during evacuationstep c), said treatment consisting in a filtration of the water from thedust or a replacement of the water itself.
 40. Method according to claim32, comprising a mixing step e) of the molten steel at least during theevacuation step c).